In order to meet the growing demand for petroleum products there is greater utilization of sour crudes, which when combined with tighter environmental legislation regarding the concentration of nitrogen and sulfur within fuel, leads to accentuated refining problems. The removal of sulfur (hydrodesulfurization—HDS) and nitrogen (hydrodenitrification—HDN) containing compounds from fuel feed stocks is targeted during the hydrotreating steps of refining and is achieved by the conversion of organic nitrogen and sulfur to ammonia and hydrogen sulfide respectively.
Since the late 1940s the use of catalysts containing nickel (Ni) and molybdenum (Mo) or tungsten (W) have demonstrated up to 80% sulfur removal. See for example, V. N. Ipatieff, G. S. Monroe, R. E. Schaad, Division of Petroleum Chemistry, 115th Meeting ACS, San Francisco, 1949. For several decades now there has been an intense interest directed towards the development of materials to catalyze the deep desulfurization, in order to reduce the sulfur concentration to the ppm level. Some recent breakthroughs have focused on the development and application of more active and stable catalysts targeting the production of feeds for ultra low sulfur fuels. Several studies have demonstrated improved HDS and HDN activities through elimination of the support such as, for example, Al2O3. Using bulk unsupported materials provides a route to increase the active phase loading in the reactor as well as providing alternative chemistry to target these catalysts.
More recent research in this area has focused on the ultra deep desulfurization properties achieved by a Ni—Mo/W unsupported ‘trimetallic’ material reported in, for example, U.S. Pat. No. 6,156,695. The controlled synthesis of a broadly amorphous mixed metal oxide consisting of molybdenum, tungsten and nickel, significantly outperformed conventional hydrotreating catalysts. The structural chemistry of the tri-metallic mixed metal oxide material was likened to the hydrotalcite family of materials, referring to literature articles detailing the synthesis and characterization of a layered nickel molybdate material, stating that the partial substitution of molybdenum with tungsten leads to the production of a broadly amorphous phase which, upon decomposition by sulfidation, gives rise to superior hydrotreating activities.
The chemistry of these layered hydrotalcite-like materials was first reported by H. Pezerat, contribution à l'étude des molybdates hydrates de zinc, cobalt et nickel, C. R. Acad. Sci., 261, 5490, who identified a series of phases having ideal formulas MMoO4.H2O, AHM2O− (MoO4)2.H2O, and A2-x(H3O)xM2O(MoO4)2 where A can be NH4+, Na+ or K+ and M can be Zn2+, Co2+ or Ni2+.
Pezerat assigned the different phases he observed as being Φc, Φy or Φy and determined the crystal structures for Φx and Φy, however owing to a combination of the small crystallite size, limited crystallographic capabilities and complex nature of the material, there were doubts raised as to the quality of the structural assessment of the materials. During the mid 1970s, Clearfield et al attempted a more detailed analysis of the Φx and Φy phases, see examples A. Clearfield, M. J. Sims, R. Gopal, Inorg. Chem., 15, 335; A. Clearfield, R. Gopal, C. H. Saldarriaga-Molina, Inorg. Chem., 16, 628. Single crystal studies on the product from a hydrothermal approach allowed confirmation of the Φx structure, however they failed in their attempts to synthesize Φy and instead synthesized an alternative phase, Na—Cu(OH)(MoO4), see A. Clearfield, A. Moini, P. R. Rudolf, Inorg. Chem., 24, 4606.
The structure of Φy was not confirmed until 1996 when by Ying et al. Their investigation into a room temperature chimie douce synthesis technique in the pursuit of a layered ammonium zinc molybdate led to a metastable aluminum-substituted zincite phase, prepared by the calcination of Zn/Al layered double hydroxide (Zn4Al2(OH)12CO3.zH2O). See example D. Levin, S. L. Soled, J. Y. Ying, Inorg. Chem., 1996, 35, 4191-4197. This material was reacted with a solution of ammonium heptamolybdate at room temperature to produce a highly crystalline compound, the structure of which could not be determined through conventional ab-initio methods. The material was indexed, yielding crystallographic parameters which were the same as that of an ammonium nickel molybdate, reported by Astier, see example M. P. Astier, G. Dji, S. Teichner, J. Ann. Chim. (Paris), 1987, 12, 337, a material belonging to a family of ammonium-amine-nickel-molybdenum oxides closely related to Pezerat's materials. Astier did not publish any detailed structural data on this family of materials, leading to Ying et al reproducing the material to be analyzed by high resolution powder diffraction in order to elucidate the structure. Ying et al named this class of materials ‘layered transition-metal molybdates’ or LTMs.